IP routing redundancy is designed to allow for transparent fail-over at the first-hop IP router. Well, first off, let’s briefly recall the concept of a default gateway: a default gateway is nothing else than the first hop for packets from a particular LAN (or VLAN to be more accurate) that need to reach a remote network; the router is able to forward such packets as long as its routing table keeps a route to the intended remote network or a default route is present. This means that if that first hop ever goes down, that particular network will become incapable to communicate to the outside world and only local communication across the switched domain will be possible, which in most cases, is of little use.
Having stated the problem, there’s a solution to it, fortunately. First Hop Redundancy protocols will allow default gateway redundancy, meaning, having more than one default gateway enabled, in the event of a router failure there’s a backup device that will kick in and almost transparently to users, continue to forward traffic to remote networks, thus avoiding the situation of isolation depicted in the above paragraph.
There are three first hop redundancy protocols that could be used for this purpose:
- Hot Standby Router Protocol (HSRP)
- Virtual Router Redundancy Protocol (VRRP)
- Gateway Load Balancing Protocol (GLBP)
We will explore more about FHRP types in next blogpost.
